Ligands and cationic complexes thereof with technetium-99m

ABSTRACT

This invention concerns novel bidentate ligands L having two phosphorus or arsenic atoms. Cationic complexes of the ligands with Technetium-99m, e.g. having the formula [TcO 2  L 2  ] + , are useful for body imaging, particularly myocardial imaging. The ligands have the general formula 
     
         Y.sub.2 QZQY.sub.2 
    
     where Q is phosphorus or 
     Z is a --CC-- or --CCC-- or --COC-- groups, 
     the four groups Y are all C1-C8 saturated hydrocarbon or fluorohydrocarbon with 1-3 ether oxygen atoms.

This invention relates to cationic complexes of technetium-99m (Tc-99m) useful as body imaging agents, and to bifunctional ligands useful in preparing such complexes.

Radiopharmaceuticals may be used as diagnostic or therapeutic agents by virtue of the physical properties of their constituent radionuclides. Thus, their utility is not based on any pharmacologic action. Most clinically used drugs of this class are diagnostic agents incorporating a gamma-emitting nuclide which because of physical or metabolic properties of its coordinated ligands, localises in a specific organ after intravenous injection. The resultant images can reflect organ structure or function. These images are obtained by means of a gamma camera that detects the distribution of ionising radiation emitted by the radioactive molecules. The principal isotope currently used in clinical diagnostic nuclear medicine is metastable technetium-99m (t1/2 6 hours).

It is well established that neutral bidentate ligands of the general type R₂ Q(CH₂)_(n) QR₂ (where Q may be phosphorus or arsenic, and n is 2 or 3) form stable well characterised cationic complexes with ⁹⁹ Tc and ^(99m) Tc.sup.[1]. Several patents, including U.S. Pat. Nos. 4,481,184, 4,387,087, 4,489,054, 4,374,821, 4,451,450 and 4,526,776 describe various ligand species in which the coordinating atoms are phosphorus or arsenic, with predominantly alkyl and/or aryl substituents.

DE 1917884 mentions the compound (CH₃ OCH₂ CH₂)₂ P --CH₂ CH₂ --P(CH₂ OCH₃)₂ for use as a catalyst.

U.S. Pat. No. 4,916,214 concerns neutral bidentate donor ligands based on phosphorus or arsenic and which contain ether linkages; and cationic complexes of Tc-99 comprising these ligands. It was found that such complexes show suprising properties which may make them superior body imaging agents, particularly heart imaging agents, to comparable complexes not containing ether linkages. The present invention is concerned with a specific class of ligands, and their associated complexes with Tc-99m, within this general field.

In a first aspect, the invention provides a ligand having the formula Y₂ QZQY₂, where each Q is phosphorus or arsenic,

Z is a --CC-- or --CCC-- or --COC-- chain or o-phenylene which may be substituted by C1-C4 alkoxy or alkoxyalkyl or spirocyclic ether,

the groups Y may be the same or different and each is a saturated hydrocarbon or saturated fluorohydrocarbon which contains from 2 to 8 carbon atoms and from 1 to 3 ether oxygen atoms, provided that the four groups Y together contain a total of at least 13 carbon atoms. Preferably each group Y has the formula

    --[(CH.sub.2).sub.m --(CR.sup.1 R.sup.2).sub.1 --O].sub.n R.sup.3

where

l is 0 or 1,

m is 1 to 6,

provided that (l+m) is 1 to 6,

n is 1 to 3,

where l is 0, R³ is --C₂ H₅ or i--C₃ H₇ or --CH₂ --CH_(x) F_(3-x)

where x is 0 to 2, or where l is 1, R1 and/or R² is --CH₃ or H and R³ is --CH₃ or --C₂ H₅

Preferably l is 0, m is 1 to 3, n is 1 and R³ is --C₂ H₅. A preferred example is the ligand called hereinafter P53, in which

Q is phosphorus,

Z is --C₂ H₄ --, and

each Y is --C₂ H₄ OC₂ H₅,

thus having the formula ##STR1##

In other preferred groups of ligands:

Each group Y comprises a 5- or 6- membered ring of C and O atoms.

Each group Y has the formula:

    --C.sub.m H.sub.2m-1 ((CH.sub.2).sub.1 OR.sup.4).sub.2

where

m is 1 to 6

l is 0 or 1, such that (1+m) is 1 to 6,

R⁴ is C1 to C3 alkyl or fluoroalkyl.

Q is phosphorus, and the two groups Y attached to each Q are different and are selected from those having the formula

    --(CH.sub.2).sub.m OR.sup.5

where m is 1 to 3, and R⁵ is C1 to C4 alkyl or fluoroalkyl.

The ligands are preferably symmetrical about Z. In all cases, the four groups pendant to the phosphorus (or arsenic) atoms all contain ether oxygen atoms.

These ligands may be made by methods generally as described in the aforesaid U.S. Pat. No. 4,916,214. Ligands of the type Y₂ QZQY₂ where all the groups Y are the same may be made by reacting one mole of the known compound bis(diphosphino)-ethane with four moles of an ether of appropriate formula containing an ethylenically unsaturated group. Symmetrical ligands where the groups Y are different may be made by reacting a known compound of the type Y(H)QZQ(H)Y with an alkyllithium and then reacting the product with a compound of the type YX where X is a monodentate ligand for Tc such as halogen.

The cationic complexes of Tc-99m may be prepared by methods known in the art. For example:

Complexes of the type [TcO₂ L₂ ]⁺ A⁻ may be prepared by methods as described in U.S. Pat. No. 4,374,821.

Complexes of the type [TcL₃ ]⁺ A⁻ may be prepared by methods as described in U.S. Pat. No. 4,481,184.

Complexes of the type [TcCl₂ L₂ ]⁺ may be prepared by methods as described in U.S. Pat. Nos. 4,387,087 and 4,489,054.

In particular, the ligands are easily labelled to provide 1 - step room temperature syntheses of Tc-99m complexes. Once formed, the complexes are stable in aqueous media in forms which are suitable for administration. Complexes which have been prepared and shown to have particularly favourable properties for use as myocardial imaging agents include those believed to have the following formulae (^(99m) Tc is abbreviated to Tc):

a) [TcL₂ O₂ ]⁺, where L is the ligand P53; and

b) [TcCl₂ L₂ ]⁺ where L is the ligand P53.

The invention includes the complexes, which can be prepared as described in the Examples below, and which may or may not have these formulae.

Although this invention is concerned with results rather than with mechanisms, applicants offer the following as a possible explanation of mechanisms. Broadly, for compounds of similar structure, there is a relationship between lipophilicity and protein binding. Compounds of high lipophilicity are more strongly bound to proteins than compounds of low lipophilicity. For ^(99m) Tc cations, the effect of high protein binding is that they remain a long time in circulation, so that the image of the heart muscle at convenient imaging times post-injection is obscured by the blood pool activity. A further generally observed tendency for the more highly lipophilic cations is that they possess slow clearance through the hepatobiliary system, so that heart imaging can be impaired by liver activity.

Substantially increasing the hydrophilicity of a ^(99m) Tc complex has the desired effect reducing protein binding but also reduces heart uptake. It appears that there is, however, a region of intermediate lipophilicity where the heart uptake is retained and there is also absent, or sufficiently weak, protein binding to permit rapid clearance from blood. This can be illustrated by reference to the complexes noted above for TcL₂ O₂ ⁺ (L=P53), a combination of the quite polar oxygens of the TcO₂ ⁺ core (2 per molecule) with the eight ethoxy ether functions of the ligand, can achieve the desired combination of good heart uptake with low protein binding. By contrast, a complex of the type a) in which the ligand is an isomer of P53 in which each group Y is --C₃ H₆ OCH₃ (compound 2), has low heart uptake and substantially higher hydrophilicity, as indicated by a high urinary output.

It appears that there may be a ranking in contribution to polarity of TcO greater than MeO greater than EtO. By means of this background understanding, it is possible to achieve the required lipophilic/hydrophilic balance through the additive hydrophilic effects of the oxygen substituents balancing out the lipophilic effects of the hydrocarbon moieties in the molecule.

The following examples illustrate the invention.

EXAMPLE 1 Ligand Synthesis (EtOC₂ H₄)₂ P--C₂ H₄ --P(C₂ H₄ OEt)₂ (P53, Compound 1)

All reactions and manipulations were performed under vacuo or oxygen-free nitrogen atmosphere. Solvents were dried, and degassed by nitrogen purge prior to use. α-Azo-isobutyronitrile (AIBN) and ethyl vinyl ether were obtained from BDH and Aldrich respectively. Bis(diphosphino)-ethane was prepared according to the literature (1).

¹ H nmr and ³¹ P[¹ H] nmr spectra were obtained from a Jeol 270 MHz machine Samples were dissolved in CDCl₃. ¹ H nmr was referenced internally to TMS and ³¹ P[¹ H] nmr was referenced externally to H₃ PO₄.

Reaction Scheme ##STR2##

A Fischer pressure-bottle equipped with a teflon stirring bar, was charged with ethyl vinyl ether (5 cm³, 52.3 mmol), bis(diphosphino)ethane (1 cm³, 10 mmol) and α-azo-isobutyronitrile (0.1 g, 0.61 mmol). The reaction mixture was then stirred and heated to 75° for 16 hours. After cooling back to room temperature, the viscous liquid was transferred to a 50 cm³ round-bottomed flask. Removal of volatile materials was performed by heating under vacuum. The non-distillable material obtained is pure by nmr. Yield: 3.0 g, 80%. ¹ H nmr (CDCl₃):

    ______________________________________                                         (ppm)   Integration   Assignment                                               ______________________________________                                         1.12    12H           Doublet of triplet,                                                            J = 1.16 Hz, 7.15 Hz,                                                          OCH.sub.2 C .sub.-- H.sub.3                              1.51    4H            Broad multiplet, PC.sub.2  .sub.-- H.sub.4 P             1.7     8H            Broad triplet, J = 7.4 Hz                                                      PC .sub.-- H.sub.2 CH.sub.2 OEt                          3.4     8H            Doublet of quartet,                                                            J = 1.16 Hz, 7.15 Hz,                                                          OC .sub.-- H.sub.2 CH.sub.3                              3.49    8H            Broad multiplet,                                                               PCH.sub.2 C .sub.-- H.sub.2 OEt                          ______________________________________                                          .sup.31 P[.sup.1 H] nmr δ = -33.17 ppm                             

Compounds 2 to 38 were prepared similarly and were characterised by ¹ H and ³¹ P NMR (see Table 1). The required unsaturated ether precursors are either commercially available or literature compounds. The preparation of vinyl ethers has been reviewed (reference 2). The most widely applicable methods of synthesis are KOH elimination from the corresponding β-haloether (reference 3) and vinyl exchange (reference 4). Allylic ethers are readily prepared using phase transfer methods (references 5, 6).

Reference 1=Inorganic Synthesis, Vol 14, 10.

2. P. Fischer in "The Chemistry of Ethers, Crown Ethers, Hydroxy Compounds and their Sulphur Analogues". Part 2 (1980). S. Patai (Ed.), p. 761.

3. R. E. Ireland and D. Habich, Chem. Ber., 114, 1418 (1981).

4. W. H. Watanabe and L. E. Conlon, J. Am. Chem. Soc., 79, 2828 (1957).

5. A. Merz, Ang. Chem. Int. Ed. Engl., 12, 846 (1973).

6. B. Boutevin et al., J. Fluorine Chem., 35, 399 (1987).

EXAMPLE 2 ##STR3##

Materials

Saline=3.5 ml

"P53"=10 μl

^(99m) Tc O₄.sup.(-) Na⁺ generator elute 1.5 ml at 1.19 GBq/ml

Method

These constituents were placed in a septum-sealed glass vial under nitrogen and stood at room temperature for 30 minutes (pH=6.8). The resulting solution was submitted to various analytical techniques, summarised as below:

Chromatography Data

The resulting solutions (above) contain no colloid or free ^(99m) TcO₄.sup.(-), and indicates that the technetium complex is present in solution in approximately 92% yield.

    ______________________________________                                         Saline                 rf = 0.00                                               Methylethylketone      rf = 0.70                                               Acetonitrile/water 50:50                                                                              rf = 1.00                                               ______________________________________                                    

HPLC Data

The complex elutes as a sharp peak at approximately 7.2 minutes

Gel Electrophoresis Data

The complex moved as a single band towards the cathode rf=-0.44 (-indicating movement towards cathode).

Biodistribution Results

See Tables 2 and 3.

EXAMPLE 3 Plasma binding studies

1 ml sample of guinea pig or human plasma or saline (control) were added to 2 ml Eppendorf tubes. To these were then added 100 ul aliquots of the ^(99m) Tc preparation being studied. Mixing was performed by inverting the tubes several times. A 50 ul sample was then taken for counting and the remainder of the sample (1050 ul) was quantitatively transferred to the top of the PD10 column. The sample was allowed to enter the column matrix and then washed in with 1.5 mls saline. Initially a 2.5 ml aliquot was collected for counting, (dead volume) subsequently 0.5 ml aliquots were collected until a total volume of 16 mls had been used to elute the column. Routinely, for a complex which does not bind non-specifically, i.e. irreversibly, to the components of the column, this treatment is sufficient to quantitatively elute all activity loaded onto the column. Collected fractions were counted in a γ well counter and a graph plotted of percentage recovery was calculated from initial preloading sample and cumulative counts.

The percent recovery of Tc99m activity obtained when using complex a) based on the P53 ligand was as follows:

saline (control)--90.82%

human plasma--65.23%

guinea pig serum--90.34%.

Though lower than the other two, the recovery figure from human serum is regarded as satisfactory and as indicating no significant binding of the complex a) to any component of human plasma.

EXAMPLE 4

Tc-99m complexes of formula [TcO₂ L₂ ]⁺ were made by the general method of Example 2, but using the compounds 2 to 38 in place of compound 1 (P53). Rat biodistribution properties of the resulting complexes are reported in Table 4.

EXAMPLE 5 Technetium III diphosphine dichloro-complex [TcCl₂ L₂ ]⁺ ##STR4## SYNTHESIS

Materials

    ______________________________________                                         FeCl.sub.3.6H.sub.2 O                                                                       5      mg                                                         P53          10     μl                                                      EGTA         15     mg                                                         NaCl         100    mg                                                         EtOH         2      ml                                                         .sup.99m TcO.sub.4.sup.- (aq)                                                               0.4    ml ˜  4 BGq Generator Eluate                         Saline       2.6    ml                                                         ______________________________________                                          EGTA = Ethylene glycolO,Obis(2-aminoethyl)-N,N,N',Ntetraacetic acid      

A purple ethanolic solution of FeCl₃ and P53 was prepared, and added to an aqueous solution containing the remaining reactants. The resulting mixture was heated at 120° C. for 60 minutes, changing colour to pale yellow. The products were submitted to various analytical techniques, summarised below:

Chromatography

The resulting solutions (above) contain no colloid or free ^(99m) TcO₄ ⁻, indicating that the technetium complex is present in solution in approximately 85-95% yield.

    ______________________________________                                         Saline               rf = 0.10                                                 Methylethyl ketone   rf = 0.75                                                 Acetonitrile/Water (50:50)                                                                          rf = 0.9 (broad)                                          ______________________________________                                    

HPLC Data:

The complex elutes as a sharp peak at 9.8 minutes (<10% impurity peak at 8.9 minutes).

Gel Electrophoresis Data:

The complex moves as a single species towards the cathode rf=-0.42 (-indicating movement towards cathode).

Biodistribution Results:

Table 5 In Rats--The Tc^(III) -P53 complex shows good heart uptake in rats and guinea pigs, good

Table 6 In Guinea Pigs--blood and liver clearance, but washes out of the respective hearts.

Plasma Binding Results:

This complex shows a possible slight interaction with human plasma (Recovery 93.8%).

                                      TABLE 1                                      __________________________________________________________________________     Y.sub.2 QZQY.sub.2                                                                               .sup.31 P                                                    No/               NMR .sup.1 H NMR; δ (ppm)                              Cmpd Y            δ/ppm                                                                        CHP,CH.sub.2 P                                                                        OCH.sub.2                                                                           OCH  OMe  OCH.sub.2 C .sub.--H.sub.3                                                           CH.sub.2 C .sub.--H.sub.                                                       2    Other                   __________________________________________________________________________     For Q = P, Z = CH.sub.2 CH.sub.2                                                1   CH.sub.2 CH.sub.2 OEt                                                                       -33.1                                                                              1.7(m) 3.5(m)         1.1(t)                                                   1.5(m) 3.4(q)                                             2   CH.sub.2 CH.sub.2 OMe                                                                       -33.0                                                                              1.77(m)                                                                               3.56(m)   3.34(s)                                                       1.58(m)                                                   3   CH.sub.2 CH.sub.2 CH.sub.2 OMe                                                              -28.5                                                                              1.75(m)                                                                               3.3(t)    3.2(s)     1.48(m)                                            1.55(m)                                                   4   CH.sub.2 CH.sub.2 CH.sub.2 OEt                                                              -26.0                                                                              1.8(m) 3.4(m)         1.2(t)                                                                               1.4(m)                                             1.6(m)                                                    5   CH.sub.2 OEt -30.6                                                                              1.78(m)                                                                               3.6(q)         1.3(t)                                                          3.9(m)                                             6   CHCHOEt      -55.1                                                                              6.47   3.9(q)                                                                              4.61      1.3(t)                                                   2.75(m)                                                   7   CH.sub.2 CH.sub.2 OCH.sub.2 CH.sub.2 OMe                                                    -33.0                                                                              1.73(t)                                                                               3.5(m)    3.3(s)                                                        1.5(m)                                                    8   CH.sub.2 CH.sub.2 OCH.sub.2 CH.sub.2 OEt                                                    -34.0                                                                              1.7(t) 3.5(m)    1.15(t)                                                       1.5(m)                                                    9   CH.sub.2 CH.sub.2 OCF.sub.2 CF.sub.2 H                                                      -32.1                                                                              1.85(t)                                                                               4.1(m)                    CF.sub.2 H                                    1.55(t)                          5.62(tt)                10   CH.sub.2 CH.sub.2 CH.sub.2 OCH.sub.2 CF.sub.3                                               -26.9                                                                              1.6(m) 3.75(q)                                                                 1.5(m) 3.6(t)                                            11   CH.sub.2 CHMeOMe                                                                            -34.6                                                                              1.7(m)      3.4(m)                                                                              3.3(s)          Me 1.08(d)                                    1.5(m)                                                   12   CH.sub.2 CHEtOMe                                                                            -25.3                                                                              1.57(m)                                                                               3.2(m)    3.4(s)                                                                              0.98(t)                            13   CH.sub.2 CHMeOEt                                                                            -33.6                                                                              1.70(m)                                                                               3.30(m)                                                                             4.50(m)   1.12(t)    Me 1.18(d)                                -34.4                                                                              1.48(m)                                                  14   CH.sub.2 CHMeCH.sub.2 OMe                                                                   -37.1                                                                              1.7(m) 3.2(m)    3.3(s)          Me 0.98(d)                                    1.5(m)                                                   15   CH.sub.2 CH.sub.2 CHMeOMe                                                                   -25.3                                                                              1.42(m)     3.3(m)                                                                              3.2(s)          Me 1.08(d)              16   CH.sub.2 CH.sub.2 CMe.sub.2 OMe                                                             - 22.7                                                                             1.5(m)           3.1(s)     1.15(m)                                                                             CMe.sub.2                                     1.3(m)                           1.09(s)                 17   CH.sub.2 CH(CH.sub.2 OMe).sub.2                                                             -35.8                                                                              1.8(m) 3.65(m)   3.2(s)          CH.sub.2 CH                                   1.37(m)                                                                               3.3(m)                    1.1(m)                  18   CH.sub.2 CH(CH.sub.2 OEt).sub.2                                                             -35.7                                                                              1.9(m) 3.4(m)         1.1(t)     CH.sub.2 CH                                   1.4(m)                           1.15(m)                 19   CH.sub.2 CH.sub.2 CH(OMe).sub.2                                                             -25.3                                                                              1.8(m)      4.43(t)                                                                             3.38(s)                                                       1.6(m)                                                   20   CH.sub.2 CH.sub.2 CH(OEt).sub.2                                                             -25.3                                                                              1.7(m) 3.5(m)                                                                              4.45(t)   1.15(t)                                                  1.4(m)                                                   21                                                                                   ##STR5##    -34.3 -35.2                                                                        1.75(d) 1.60(m)                                                                       3.9(brs) 3.4(brs)                                                                   4.1(brs)             OCH.sub.2 O                                                                    4.70,4.85(s)            22                                                                                   ##STR6##    -36.0 (m)                                                                          1.75(d)                                                                               3.55(m)                                                                             4.1(m)               CMe.sub.2 1.40,1.25                                                            (s)                     23                                                                                   ##STR7##    -8.9 (m)                                                                           2.13(m)                                                                               3.75(m)              1.6(m)                       24                                                                                   ##STR8##    -34.5 (m)                                                                          1.5(m) 1.9(m)                                                                         3.80(m)                                                                             3.55(m)         1.7(m)                       25                                                                                   ##STR9##    -39.5 (m)                                                                          1.85 1.7                                                                              3.5(t)               1.45(m)                                                                             CH.sub.2 C                                                                     .sub.--H 1.56(m)        26                                                                                   ##STR10##   -33.4                                                                              1.5(m) 3.6(m)                                                                              3.2(m)                                       27   CH.sub.2 CH.sub.2 OMe                                                                       -32.4                                                                              2.0(m) 3.6(m)    3.17(s)                                      CH.sub.2 OMe     1.8(m)           3.19(s)                                 28   CH.sub.2 CH.sub.2 OMe                                                                       -33.4                                                                              1.85(t)                                                                               3.4(q)    3.2(s)                                                                              1.2(t)                                  CH.sub.2 CH.sub.2 OEt                                                                           1.7(t) 3.6(m)                                            29   CH.sub.2 CH.sub.2 OMe                                                                       -32.9                                                                              1.7(m) 3.5(m)    3.27(s)                                                                             0.89(t)                                 CH.sub.2 CH.sub.2 OPr                                                                           1.5(m) 3.3(m)                                            30   CH.sub.2 CH.sub.2 OMe                                                                       -30.7                                                                              1.7(m) 3.5(m)    3.37(s)                                                                             1.1(t)                                   CH.sub.2 CH.sub.2 CH.sub.2 OEt                                                                 1.5(m)                                                   31   CH.sub.2 CH.sub.2 OEt                                                                       -32.9                                                                              1.7(m) 3.48(m)        1.1(t)     CMe.sub.2 1.18(m)            CH.sub.2 CH.sub.2 OiPr                                                                          1.5(t) 3.37(q)                                           32   CH.sub.2 CH.sub.2 OEt                                                                       -32.8                                                                              1.7(m) 3.5(m)         1.12(t)    CH.sub.2 F                                                                     4.5(dt)                      CH.sub.2 CH.sub.2 OCH.sub.2 CH.sub.2 F                                                          1.5(t) 3.41(q)                                           For Q = P, Z = CH.sub.2 CH.sub.2 CH.sub.2                                      33   CH.sub.2 OEt -35.2                                                                              1.78(m)                                                                               3.84(m)        1.33(m)                                                         3.64(q)                                           34   CH.sub.2 CH.sub.2 OEt                                                                       -39.0                                                                              1.8(m) 3.6(m)         1.15(t)                                                  1.55(m)                                                                               3.4(q)                                            35                                                                                   ##STR11##   -32.0 (m)                                                                          1.8(m) 3.65(m)                                                                             3.85(m)         1.5(m) 1.2(m)                For Q = P, Z = CH.sub.2 CH(OMe)CH.sub.2                                        36  CH.sub.2 CH.sub.2 OEt                                                                        -40.9                                                                              1.8(m) 3.5(m)    3.37(m)                                                                             1.1(t)                             For Q = P, Z =  -o phenylene                                                   37  CH.sub.2 CH.sub.2 OEt                                                                        -45.1                                                                              2.3(m) 3.65(m)        1.1(t)     Arom 7.2(m)                                          3.25(q)                                           38  CH.sub.2 CH.sub.2 CH.sub.2 OEt                                                               -38.0                                                                              1.9(m) 3.5(m)         1.15(t)                                                                              1.8(m)                                                                              Arom 7.3(m)                                          3.4(q)                                            __________________________________________________________________________

                  TABLE 2                                                          ______________________________________                                         Animal Biodistribution Data in Rat [TcO.sub.2 (P53).sub.2 ].sup.+               ##STR12##                                                                     Time p.i.  2 min              60 min                                           in vivo    Mean    Std. dev.  Mean  Std. dev.                                  ______________________________________                                                  % Injected dose/organ                                                 Heart      1.66    0.11       1.68  0.13                                       Blood      2.25    0.43       0.28  0.00                                       Muscle     31.6    9.3        31.6  2.4                                        Lung       1.44    0.05       0.65  0.20                                       Liver      14.1    1.3        1.62  0.34                                       Liver + GI 37.8    2.9        42.5  3.5                                        Kidney + Urine                                                                            13.2    1.8        9.29  2.53                                       Brian      0.03    0.05       0.02  0.01                                                Counts/Gram Ratio                                                     Heart/Blood                                                                               10.5    1.8        86.4  9.8                                        Heart/Muscle                                                                              5.94    2.57       5.74  0.93                                       Heart/Liver                                                                               1.52    0.08       14.4  1.8                                        Heart/Lung 2.0     0.3        4.3   1.9                                        ______________________________________                                    

                  TABLE 3                                                          ______________________________________                                         Animal Biodistribution Data in Guinea Pig [ TcO.sub.2 (P53).sub.2              ].sup.+                                                                         ##STR13##                                                                     Time p.i.  2 min              60 min                                           in vivo    Mean    std. dev.  Mean  std. dev.                                  ______________________________________                                                  % Injected dose/organ                                                 Heart      1.33    0.14       0.99  0.12                                       Blood      3.11    0.80       0.41  0.07                                       Muscle     30.0    4.4        43.4  28.4                                       Lung       1.14    0.19       0.47  0.03                                       Liver      11.6    2.5        2.13  0.56                                       Liver + GI 38.7    4.1        44.6  2.8                                        Kidney + Urine                                                                            16.7    1.2        16.1  1.4                                        Brain      --      --         --    --                                                  Counts/Gram Ratio                                                     Heart/Blood                                                                               10.9    2.7        61.9  12.0                                       Heart/Muscle                                                                              6.12    0.44       4.10  2.09                                       Heart/Liver                                                                               1.90    0.87       7.51  2.40                                       Heart/Lung 1.2     0.2        2.0   0.2                                        ______________________________________                                    

                                      TABLE 4                                      __________________________________________________________________________     RAT BIODISTRIBUTION OF TcO.sub.2 (PHOSPHINE).sub.2.sup.+                       % Injected dose/organ       Counts/g ratio                                     HEART       BLOOD   LIVER   HEART/BLOOD                                                                              HEART/LIVER                              Cmpd                                                                               2 min                                                                              60 min                                                                             2 min                                                                              60 min                                                                             2 min                                                                              60 min                                                                             2 min                                                                               60 min                                                                              2 min                                                                               60 min                              __________________________________________________________________________      1  1.66                                                                               1.68                                                                               2.25                                                                               0.28                                                                               14.1                                                                               1.62                                                                               10.5 86.4 1.52 14.4                                 2  0.66                                                                               0.17                                                                               19.85                                                                              0.54                                                                               5.90                                                                               2.37                                                                               0.44 5.4  1.57 1.17                                 3  0.44                                                                               0.09                                                                               18.8                                                                               0.77                                                                               3.62                                                                               1.35                                                                               0.35 1.82 1.71 0.88                                 4  1.00                                                                               0.96                                                                               7.46                                                                               0.73                                                                               18.8                                                                               6.96                                                                               1.96 20.0 0.81 2.13                                 5  0.54                                                                               0.27                                                                               12.4                                                                               1.63                                                                               33.1                                                                               8.85                                                                               0.68 2.6  0.20 0.37                                 6  0.58                                                                               0.46                                                                               15.6                                                                               3.26                                                                               44.3                                                                               30.6                                                                               0.61 2.2  0.18 0.19                                 8  0.54                                                                               0.42                                                                               11.0                                                                               1.69                                                                               19.0                                                                               12.0                                                                               0.72 4.0  0.41 0.46                                10  0.43                                                                               0.14                                                                               40.5                                                                               7.59                                                                               35.2                                                                               48.8                                                                               0.17 0.3  0.16 0.04                                11  0.31                                                                               0.62                                                                               9.45                                                                               1.93                                                                               20.3                                                                               12.0                                                                               1.27 5.70 0.57 0.75                                12  0.70                                                                               0.48                                                                               11.7                                                                               1.63                                                                               25.2                                                                               9.72                                                                               1.04 4.7  0.39 0.63                                13  1.21                                                                               0.96                                                                               7.17                                                                               1.77                                                                               22.0                                                                               10.6                                                                               2.54 7.9  0.70 1.25                                14  0.41                                                                               0.05                                                                               15.4                                                                               1.02                                                                               5.80                                                                               1.54                                                                               0.41 0.7  0.98 0.39                                15  0.62                                                                               0.51                                                                               12.2                                                                               0.67                                                                               8.22                                                                               2.24                                                                               0.70 9.4  1.01 2.48                                16  0.79                                                                               0.54                                                                               11.4                                                                               1.58                                                                               19.2                                                                               7.71                                                                               1.01 4.69 0.58 0.96                                17  1.14                                                                               0.86                                                                               5.86                                                                               1.03                                                                               17.1                                                                               7.88                                                                               2.99 13.9 0.92 1.59                                18  0.58                                                                               0.41                                                                               24.2                                                                               5.78                                                                               35.1                                                                               36.0                                                                               0.37 1.0  0.24 0.14                                19  0.46                                                                               0.03                                                                               18.8                                                                               0.88                                                                               3.27                                                                               0.67                                                                               0.37 0.6  1.29 0.76                                20  1.01                                                                               0.81                                                                               7.14                                                                               1.52                                                                               24.50                                                                              14.90                                                                              1.95 7.9  0.50 0.69                                21  0.37                                                                               0.04                                                                               18.6                                                                               1.14                                                                               9.83                                                                               1.95                                                                               0.33 0.5  0.58 0.26                                22  0.99                                                                               1.00                                                                               10.4                                                                               1.23                                                                               15.7                                                                               3.41                                                                               1.20 12.3 0.79 3.99                                24  1.82                                                                               1.42                                                                               3.92                                                                               0.55                                                                               16.4                                                                               8.56                                                                               6.98 39.7 1.74 2.62                                25  0.40                                                                               0.04                                                                               18.10                                                                              1.51                                                                               5.74                                                                               1.60                                                                               0.34 0.41 0.94 0.34                                28  0.82                                                                               0.69                                                                               9.0 0.73                                                                               11.0                                                                               2.94                                                                               1.62 15.5 1.28 3.93                                29  1.23                                                                               1.15                                                                               8.75                                                                               1.64                                                                               18.6                                                                               11.5                                                                               2.26 11.5 0.95 1.40                                30  0.73                                                                               0.70                                                                               9.52                                                                               0.34                                                                               7.69                                                                               1.89                                                                               1.14 27.4 1.28 4.24                                31  1.21                                                                               0.99                                                                               6.88                                                                               1.22                                                                               25.0                                                                               12.4                                                                               3.05 13.4 0.84 1.21                                32  0.85                                                                               0.81                                                                               5.85                                                                               0.38                                                                               10.8                                                                               1.78                                                                               2.31 33.2 1.27 7.46                                33  0.50                                                                               0.29                                                                               17.9                                                                               3.20                                                                               39.4                                                                               11.4                                                                               0.43 1.3  0.16 0.29                                34  0.41                                                                               0.16                                                                               19.5                                                                               4.56                                                                               29.7                                                                               10.9                                                                               0.34 0.52 0.22 0.21                                35  0.59                                                                               0.32                                                                               10.5                                                                               1.44                                                                               28.2                                                                               10.7                                                                               0.88 3.8  0.28 0.46                                36  1.05                                                                               0.73                                                                               10.8                                                                               2.18                                                                               18.8                                                                               6.64                                                                               1.45 5.69 0.73 1.62                                37  0.82                                                                               0.54                                                                               12.4                                                                               3.25                                                                               29.7                                                                               12.0                                                                               0.99 2.62 0.35 0.63                                __________________________________________________________________________

                  TABLE 5                                                          ______________________________________                                         ANIMAL BIODISTRIBUTION DATA IN RAT                                             Complex: [Technetium III dichloro-(P53).sub.2 ].sup.+ cation                   Time p.i.  2 min          60 min                                               in vivo    Mean     Std. Dev. Mean   Std. Dev.                                 ______________________________________                                                  % injected dose/organ                                                 Heart      1.51     0.20      0.16   0.01                                      Blood      5.24     0.11      0.71   0.03                                      Muscle     34.92    12.89     11.68  1.69                                      Lung       1.45     0.08      0.47   0.18                                      Liver      28.87    4.71      4.78   0.28                                      Liver + GI 41.13    5.54      61.34  0.90                                      Kidney + Urine                                                                            4.48     0.56      4.95   1.00                                               Counts/Gram ratio                                                     Heart/Blood                                                                               3.90     0.45      3.09   0.07                                      Heart/Muscle                                                                              4.68     1.40      1.41   0.20                                      Heart/Liver                                                                               0.64     0.10      0.46   0.03                                      ______________________________________                                    

                  TABLE 6                                                          ______________________________________                                         ANIMAL BIODISTRIBUTION IN GUINEA PIG                                           Complex: [Technetium III-dichloro(P53).sub.2 ].sup.+ cation                    Time p.i.  2 min          60 min                                               in vivo    Mean     Std. Dev. Mean   Std. Dev.                                 ______________________________________                                                  % injected dose/organ                                                 Heart      1.89     0.27      0.13   0.01                                      Blood      3.50     0.64      0.58   0.09                                      Muscle     61.65    19.29     15.16  2.19                                      Lung       3.11     0.46      0.30   0.04                                      Liver      13.74    2.81      3.16   0.71                                      Liver + GI 18.72    4.54      62.38  19.4                                      Kidney + Urine                                                                            6.98     1.49      3.20   1.07                                               Counts/Gram ratio                                                     Heart/Blood                                                                               12.72    1.68      5.35   1.08                                      Heart/Muscle                                                                              4.48     2.44      1.13   0.17                                      Heart/Liver                                                                               2.13     0.44      0.72   0.19                                      ______________________________________                                     

We claim:
 1. A ligand having the formula

    Y.sub.2 QZQY.sub.2

where each Q is phosphorus, Z is a --CC-- or --CCC-- chain or o-phenylene which may be substituted by C1-C3 alkoxy or alkoxyalkyl, the groups Y may be the same or different and each is a saturated hydrocarbon or saturated fluorohydrocarbon which contains from 2 to 8 carbon atoms and from 1 to 3 ether oxygen atoms, provided that the four groups Y together contain a total of at least 13 carbon atoms.
 2. A ligand as claimed in claim 1, wherein the groups Y may be the same or different and each has the formula

    --[(CH.sub.2).sub.m --(CR.sup.1 R.sup.2).sub.1 --O].sub.n R.sup.3

where 1 is 0 or 1, m is 1 to 3, provided that (1+m) is 1 to 3, n is 1 or 2, where 1 is 0, R³ is --C₂ H₅ or i--C₃ H₇ or --CH₂ --CH_(x) F_(3-x) where x is 0 to 2, or where l is 1, R¹ and/or R² is --CH₃ or H and R³ is --CH₃ or --C₂ H₅
 3. A ligand as claimed in claim 2, where m is 1 to 3 and n is
 1. 4. A ligand as claimed in claim 2, whereinQ is phosphorus, Z is --C₂ H₄ --, each Y is --C₂ H₄ OC₂ H₅.
 5. A ligand as claimed in claim 1, wherein each group Y comprises a 5- or 6- membered alicyclic ring containing 1 or 2 ether oxygen atoms.
 6. A ligand as claimed in claim 1, wherein each group Y has the formula:

    --C.sub.m H.sub.2m-1 ((CH.sub.2).sub.1 OR.sup.4).sub.2

where m is 1 to 3 l is 0 to 1, such that (l+m) is 1 to 3, R⁴ is C1 to C3 alkyl or fluoroalkyl.
 7. A ligand as claimed in claim 1, wherein Q is a phosphorus, and the two groups Y attached to each Q are different and are selected from those having the formula

    --(CH.sub.2).sub.m O R.sup.5

where m is 1 to 3, and R⁵ is C1 to C4 alkyl or fluoroalkyl.
 8. A cationic complex of technetium-99m with a ligand according to claim
 1. 9. A complex as claimed in claim 8 having the formula [TcL₂ X₂ ]⁺ or [TcL₂ O₂ ]⁺ where L is the ligand and X is a monodentate ligand for Tc.
 10. A complex as claimed in claim 8, having the formula [TcL₂ O₂ ]⁺ where L is a ligand having the formula (C₂ H₅ OC₂ H₄)₂ P--C₂ H₄ --P(C₂ H₄ OC₂ H₅)₂. 